[PATCH V5 2/5] mm: update ptep_modify_prot_commit to take old pte value as arg

[Michael Ellerman]

"Aneesh Kumar K.V" <aneesh.kumar at linux.ibm.com> writes:

> Architectures like ppc64 require to do a conditional tlb flush based on the old
> and new value of pte. Enable that by passing old pte value as the arg.

It's not actually the architecture, it's to work around a specific bug
on Power9.

> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index c89ce07923c8..028c724dcb1a 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -110,8 +110,8 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>  					continue;
>  			}
>  
> -			ptent = ptep_modify_prot_start(vma, addr, pte);
> -			ptent = pte_modify(ptent, newprot);
> +			oldpte = ptep_modify_prot_start(vma, addr, pte);
> +			ptent = pte_modify(oldpte, newprot);
>  			if (preserve_write)
>  				ptent = pte_mk_savedwrite(ptent);

Is it OK to reuse oldpte here?

It was set at the top of the loop with:

		oldpte = *pte;

Is it guaranteed that ptep_modify_prot_start() returns the old value
unmodified, or could an implementation conceivably filter some bits out?

If so then it could be confusing for oldpte to have its value change
half way through the loop.


cheers